The present invention relates to new and improved thermoelectric devices for the generation of electricity.
It has been recognized that the world supply of fossil fuels for the production of energy is being exhausted at ever increasing rates. This realization has resulted in an energy crisis which impacts not only the world's economy, but threatens the peace and stability of the world. The solution to the energy crisis lies in the development of new fuels and more efficient techniques to utilize them. To that end, the present invention deals with energy conservation, power generation, pollution, and the generation of new business opportunities by the development of new thermoelectric devices which provide more electricity.
An important part of the solution with respect to the development of permanent, economical energy conversion lies in the field of thermoelectrics wherein electrical power is generated by heat. It has been estimated that more than two-thirds of all our energy, for example, from automobile exhausts or power plants, is wasted and given off to the environment. Up until now, there has been no serious climatic effect from this thermal pollution. However, it has been predicted that as the world's energy consumption increases, the effects of thermal pollution will ultimately lead to a partial melting of the polar ice caps with an attendant increase in sea level.
The efficiency of a thermoelectric device can be expressed in terms of a figure of merit (Z) for the material forming the device, the equation defining Z is: EQU Z=S.sup.2 .sigma./K
where:
Z is expressed in units.times.10.sup.3 ; PA1 S is the Seebeck coefficient in .mu.V/.degree.C.; PA1 K is the thermal conductivity in mW/cm-.degree.C.; and PA1 .sigma. is the electrical conductivity in (.OMEGA.-cm).sup.-1
From the above, one can see that in order for a material to be suitable for thermoelectric power conversion, it must have a large value for the thermoelectric power Seebeck coefficient (S), a high electrical conductivity (.sigma.), and a low thermal conductivity (K). Further, there are two components to the thermal conductivity (K): K.sub.l, the lattice component; and K.sub.e, the electrical component. In non-metals, K.sub.l dominates and it is this component which mainly determines the value of K.
Stated in another way, in order for a material to be efficient for thermoelectric power conversion, it is important to allow carriers to diffuse easily from the hot junction to the cold junction while maintaining the temperature gradient. Hence, high electrical conductivity is required along with low thermal conductivity.
Thermoelectric power conversion has not found wide usage in the past. The major reason for this is that prior art thermoelectric materials which are at all suitable for commercial applications have been crystalline in structure. Crystalline solids cannot attain large values of electrical conductivity while maintaining low thermal conductivity. Most importantly, because of crystalline symmetry, thermal conductivity cannot be controlled by modification.
In the case of the conventional polycrystalline approach, the problems of single crystalline materials still dominate. However, new problems are also encountered by virtue of the polycrystalline grain boundaries which cause these materials to have relatively low electrical conductivities. In addition, the fabrication of these materials is also difficult to control as a result of their more complex crystalline structure. The chemical modification or doping of these materials, because of the above problems are especially difficult.
Among the best known currently existing polycrystalline thermoelectric materials are (Bi,Sb).sub.2 Te.sub.3, PbTe, and Si-Ge. The (Bi,Sb).sub.2 Te.sub.3 materials are best suited for applications in the -10.degree. C. to 150.degree. C. range with their best Z appearing at around 30.degree. C. (Bi,Sb).sub.2 Te.sub.3 represents a continuous solid solution system in which the relative amount of Bi and Sb are from 0 to 100%. The Si-Ge material is best suited for high temperature applications in the 600.degree. C. to 1000.degree. C. range with a satisfactory Z appearing at above 700.degree. C. The PbTe polycrystalline material exhibits its best figure of merit in the 300.degree. C. to 500.degree. C. range. None of these materials are well suited for applications in the 100.degree. C. to 300.degree. C. range. This is indeed unfortunate, because it is in this temperature range where a wide variety of waste heat applications are found. Among such applications are geothermal waste heat and waste heat from internal combustion engines in, for example, trucks, buses, and automobiles. Applications of this kind are important because the heat is truly waste heat. Heat in the higher temperature ranges must be intentionally generated with other fuels and therefore is not truly waste heat.
New and improved thermoelectric alloy materials have been discovered for use in the aforesaid temperature ranges. These materials are disclosed and claimed in co-pending U.S. application Ser. No. 412,306, filed Aug. 27, 1982 in the names of Tumkur S. Jayadev and On Van Nguyen for New Multiphase Thermoelectric Alloys and Method of Making Same now U.S. Pat. No. 4,447,277 issued May 8, 1984 and U.S. application Ser. No. 414,917, filed Sept. 3, 1982 in the names of Tumkur S. Jayadev, On Van Nguyen, Jamie M. Reyes, Helen Davis and Michael W. Putty for New Powder Pressed Thermoelectric Materials and Method of Making Same, which patent and application are incorporated herein by reference.
The thermoelectric materials there disclosed can be utilized in the devices herein. These materials are not single phase crystalline materials, but instead, are disordered materials. Further, these materials are multiphase materials having both amorphous and multiple crystalline phases. Materials of this type are good thermal insulators. They include grain boundaries of various transitional phases varying in composition from the composition of matrix crystallites to the composition of the various phases in the grain boundary regions. The grain boundaries are highly disordered with the transitional phases including phases of high thermal resistivity to provide high resistance to thermal conduction. Contrary to conventional materials, the material is designed such that the grain boundaries define regions including conductive phases therein providing numerous electrical conduction paths through the bulk material for increasing electrical conductivity without substantially affecting the thermal conductivity. In essence, these materials have all of the advantages of polycrystalline materials in desirably low thermal conductivities and crystalline bulk Seebeck properties. However, unlike the conventional polycrystalline materials, these disordered multiphase materials also have desirably high electrical conductivities. Hence, as disclosed in the aforementioned referenced applications, the S.sup.2 .sigma. product for the figure of merit of these materials can be independently maximized with desirably low thermal conductivities for thermoelectric power generation.
Amorphous materials, representing the highest degree of disorder, have been made for thermoelectric applications. The materials and methods for making the same are fully disclosed and claimed, for example, in U.S. Pat. Nos. 4,177,473, 4,177,474, 4,178,415 which issued in the name of Stanford R. Ovshinksky. The materials disclosed in these patents are formed in a solid amorphous host matrix having structural configurations which have local rather than long-range order and electronic configurations which have an energy gap and an electrical activation energy. Added to the amorphous host matrix is a modifier material having orbitals which interact with the amorphous host matrix as well as themselves to form electronic states in the energy gap. This interaction substantially modifies the electronic configurations of the amorphous host matrix to substantially reduce the activation energy and hence, increase substantially the electrical conductivity of the material. The resulting electrical conductivity can be controlled by the amount of modifier material added to the host matrix. The amorphous host matrix is normally of intrinsic-like conduction and the modified material changes the same to extrinsic-like conduction.
As also disclosed therein, the amorphous host matrix can have lone-pairs having orbitals wherein the orbitals of the modifier material interact therewith to form the new electronic states in the energy gap. In another form, the host matrix can have primarily tetrahedral bonding wherein the modifier material is added primarily in a non-substitutional manner with its orbitals interacting with the host matrix. Both d and f band materials as well as boron and carbon, which add multiorbital possibilities can be used as modifiers to form the new electronic states in the energy gap.
As a result of the foregoing, these amorphous thermoelectric materials have substantially increased electrical conductivity. However, because they remain amorphous after modification, they retain their low thermal conductivities making them well suited for thermoelectric applications, especially in high temperature ranges above 400.degree. C.
These materials are modified on an atomic or microscopic level with the atomic configurations thereof substantially changed to provide the above-mentioned independently increased electrical conductivities. In contrast, the materials disclosed in the aforementioned referenced applications are not atomically modified. Rather, they are fabricated in a manner which introduces disorder into the material on a macroscopic level. This disorder allows various phases including conductive phases to be introduced into the material much in the same manner as modification atomically in pure amorphous phase materials to provide controlled high electrical conductivity while the disorder in the other phases provides low thermal conductivity. These materials therefore are intermediate in terms of their thermal conductivity between amorphous and regular polycrystalline materials.
A thermoelectric device generates electricity by the establishment of a temperature differential across the materials contained therein. Thermoelectric devices generally include elements of both p-type and n-type material. In the p-type material the temperature differential drives positively charged carriers from the hot side to the cold side of the elements, while in the n-type material the temperature differential drives negatively charged carriers from the hot side to the cold side of the elements.
Thermoelectric power conversion has not found wide usage in the past not only because of material limitations but also because of device limitations. Among the device limitations are bowing or warping of device substrates, loss of broad surface contact between the device and a heat exchanger when utilized in a thermoelectric system, and temperature losses across the substrates.
Thermoelectric devices of the prior art use copper lead patterns placed upon a ceramic substrate for the attachment of thermoelectric elements thereto. In the manufacture of these devices, a second ceramic substrate having another copper lead pattern is sweated onto the thermoelectric elements. In the sweating process, a layer of solder or brazing alloy is applied to the members being joined, by methods such as stencilling a granular paste thereonto, and the assembly is heated to a temperature sufficient to fuse the solder or brazing alloy and thereby join the elements. Due to the difference in the coefficient of thermal expansion between the ceramic substrates and the copper lead patterns, there occurs a bowing or warping of the substrates during the sweating operation which causes a number of related problems.
First, because of the warping of the substrates, it is difficult, if not impossible, to obtain a good thermal connection between the elements and the copper lead patterns of the substrates. Additionally, because the ceramic substrates are brittle, the bowing or warping, if severe enough, can cause cracking of the substrates and other physical degradation of the devices. Furthermore, to be employed in a thermoelectric system, the outer surfaces of the substrates must make intimate broad surface contact with a heat exchanger. The warping or bowing of the substrates makes proper connection between the devices and a heat exchanger difficult.
To overcome these problems, the forces imparted to the substrates caused by the difference in the coefficients of thermal expansion between the copper lead patterns and the ceramic substrates are equalized by applying copper in substantially identical patterns as the lead patterns to the other side of the substrates. Unfortunately, the additional copper increases the material cost of the devices and adds extra processing steps to their manufacture. Furthermore, the degree of abatement of thermal stress obtained by this process is still not satisfactory.
During the operation of thermoelectric devices, a temperature differential is applied across the device to generate electricity. Due to the difference in the coefficient of thermal expansion between the substrates and the thermoelectric elements, loss of broad surface contact occurs between the device and the heat exchanger when utilized in a theremoelectric system. This loss of broad surface contact results in less heat transfer which translates to a lower temperature differential across the device, and lower efficiency of the device.
It has also been found that a substantial temperature drop occurs across the ceramic substrates. The voltage output and the current of the thermoelectric element is proportional to the temperature differential across the element. Therefore, the power is proportional to the square of the temperature differential, and any change in temperature differential across the elements has a substantial effect on the power output of the device. As a result, the temperature drop across the substrates reduces the temperature differential otherwise available to the elements for power generation. Further, the additional copper used to overcome the warping problems adds additional temperature losses across the substrates. These losses undesirably decrease the temperature differential across the thermoelectric elements from the temperature differential available across the devices thereby adversely decreasing the power output of the devices.
Because of these problems, thermoelectric devices of the prior art often failed during operation due to cracking of thermoelectric elements, loosening of electrical and thermal connections and fracture of the substrate. Furthermore, warpage and loss of broad surface contact results in lower heat transfer which translates into a lower temperature differential across the device and therefore, a lower overall efficiency.
In an attempt to overcome these problems, a substrateless device has been proposed. This device and method of fabrication thereof is disclosed and claimed in co-pending U.S. application Ser. No. 372,688 filed Apr. 28, 1982, now U.S. Pat. No. 4,459,428, in the name of Der-Jeou Chou for Improved Thermoelectric Device and Method of Making Same, which application is also incorporated herein by reference. The device disclosed therein has no substrates and no compensating elements. Consequently, thermal losses normally resulting from these components are eliminated. Futhermore, the disclosed device exhibits a lower degree of thermal stress because the thermoelectric generating elements are flexibly held together by a plurality of conductive interconnecting members and a potting compound filling the voids between the elements. This structure results in a device having a high degree of flexibility. Although the efficiency of this device is high, performance under many circumstances leaves much to be desired. Unfortunately, the interconnect members and the potting compound between the elements provide the device with only limited structural integrity and, consequently, it is not rugged enough under some circumstances to withstand mechanical vibration during handling, installation or use which of course limits the utility of this device.
There is, therefore, a need for a thermoelectric device which provides both structural reliability and high efficiency. More specifically, there is a need for a thermoelectric device which is capable of withstanding mechanical stresses resulting from its use in systems presenting a high degree of vibration or mechanical shock such as in internal combustion engines or the like; and which exhibits a low degree of thermal stress when a temperature gradient is applied thereto, thereby eliminating problems of damage and poor heat transfer, and which also exhibits decreased thermal losses, thereby fully exploiting the efficiencies of the new and improved thermoelectric materials previously mentioned and disclosed in U.S. Pat. No. 4,447,277 and U.S. Ser. No. 414,917. The thermoelectric devices of the instant invention satisfy these needs as will be described hereinafter; one device embodying the present invention includes one rigid substrate upon which the thermoelectric elements and their accompanying interconnect means are rigidly mounted, thereby providing a unitary, rugged structure. In addition, the device includes means for relieving thermal stress, so that substrate warpage and consequent poor heat transfer and device failure are eliminated. In addition, the presence of the stress relieving means obviates the need for the stress compensating copper bars of the prior art thereby eliminating the efficiency losses associated with such components. In accordance with another embodiment of the instant invention the other substrate can be eliminated completely thereby further decreasing thermal losses and increasing efficiencies.
As will be demonstrated hereinafter, the new and improved thermoelectric generating devices of the instant invention overcome the problems of the prior art. Mechanical strength is retained, while high efficiencies and power output are obtained as a result of efficient heat transfer and superior thermal contact due to the elimination of device warpage.